In the article a class of \(\mathbb {H}\) -valued monogenic fractional power functions defined in the reduced quaternions and depending on the parameters \(p\in \mathbb {N}_{0}\) and real \(\lambda > -1\) is constructed. These functions are an extension of the well-known class of orthogonal Appell polynomials, which is included as a special case. For the monogenic fractional powers essential properties, i.e. monogenicity, a generalized Appell property and a two-step recurrence formula, are proved and their corresponding Kelvin transforms in terms of a corresponding anti-monogenic fractional power function are given.